1. Field of the Invention
This invention relates to a process for the fabrication of a semiconductor integrated circuit device, particularly to a technique effective when applied to gate processing of MOSFET (Metal Oxide Semiconductor Field Effect Transistor) having a poly-metal gate.
2. Description of the Related Art
In Japanese Patent Application Laid-Open No. H5-152282 (which will hereinafter be called “Ohmi”), disclosed is an oxide film forming device for oxidizing a semiconductor wafer or the like with water synthesized using a catalyst such as nickel.
In pages 128–133 of Papers of the Lecture addressed at the 45-th Symposium of Semiconductor Integrated Circuit Technique held on Dec. 1–2, 1992 (which will hereinafter be called “Nakamura”), disclosed is oxide film formation under a strong reducing atmosphere which contains water vapor synthesized by a stainless catalyst.
In Japanese Patent Application Laid-Open No. S59-132136 (which will hereinafter be called “Kobayashi”), disclosed is a process, in a silicon semiconductor integrated circuit device having a refractory metal (high-melting point metal) gate, for carrying out auxiliary thermal oxidation (which will hereinafter be called “light oxidation” or “auxiliary oxidation after gate formation”) of not a gate metal itself but necessary portions other than the gate metal under a mixed atmosphere of water vapor and hydrogen after gate patterning, thereby adjusting an insulation film at the periphery below the gate.
Similarly, in Japanese Patent Application Laid-Open No. H3-119763 (which will hereinafter be called “Katada”) and Japanese Patent Application Laid-Open No. H7-94716 (which will hereinafter be called “Muraoka”), disclosed is a process for forming a gate having a three-layered structure made of tungsten, titanium nitride and polysilicon and then subjecting it to oxidation treatment under a mixed atmosphere of hydrogen, water vapor and nitrogen.
It is presumed that for a device having a circuit formed of a minute MOSFET whose gate length is 0.25 μm or smaller, such as DRAM (Dynamic Random Access Memory) available since the development of 256 Mbit (megabit), a gate processing step using a low-resistance conductive material including a metal film will become indispensable to reduce a parasitic resistance of a gate electrode.
As such a low-resistance gate electrode material, so-called poly-metal (which generally means a refractory metal formed directly or indirectly over a polycrystalline silicon electrode) having a refractory metal film stacked over a polycrystalline silicon film is regarded as promising. The poly-metal can be used not only as a gate electrode material but also as an interconnection material because of a sheet resistance as low as 2 Ω/□. According to the investigation by the present inventors, W (tungsten), Mo (molybdenum), Ti (titanium) or the like which exhibits good low resistance even in the low temperature process not higher than 800° C. and has high electro-migration resistance is used as the refractory metal. Since the stacking of such a refractory metal film directly on a polycrystalline silicon film lowers the adhesion between these two films or happens to form a high-resistance silicide layer at the interface between these two films by a high-temperature heat treatment process, the poly-metal gate is practically formed of a three-layered structure having, between the polycrystalline silicon film and the refractory metal film, a barrier layer made of a metal nitride film such as TiN (titanium nitride) or WN (tungsten nitride).
Based on the investigation by the present inventors, gate processing is conventionally carried out as follows. First, a semiconductor substrate is thermally oxidized to form a gate oxide film on its surface Although a thermally oxidized film is generally formed in a dry oxygen atmosphere, wet oxidation is employed for the formation of a gate oxide film because a defect density in the film can be reduced. In wet oxidation, a pyrogenic method is employed in which water is formed by the combustion of hydrogen in an oxygen atmosphere and then fed to the surface of a semiconductor wafer together with oxygen.
The pyrogenic method, however, is accompanied with the drawback that combustion is effected by igniting hydrogen jetted from a nozzle attached to the tip of a quartz-made hydrogen gas inlet tube, but heat generated upon combustion melts the nozzle and generates particles, which presumably become a contamination source of a semiconductor wafer. For the formation of water, therefore, a method using a catalyst without combustion is also-proposed.
The above-described Ohmi discloses a thermal oxidation apparatus having a hydrogen gas inlet tube formed at its inside from Ni (nickel) or a Ni-containing material and being equipped with means for heating the hydrogen gas inlet tube. In this thermal oxidation apparatus, Ni (or Ni-containing material) inside of the hydrogen gas inlet tube heated at 300° C. or higher is brought into contact with hydrogen to generate a hydrogen activated species and then, the hydrogen activated species is reacted with oxygen (or an oxygen-containing gas), whereby water is generated. Water is thus generated in such a catalytic manner without combustion so that neither melting of the quartz-made hydrogen gas inlet tube at its tip point nor generation of particles occurs.
Over the gate oxide film formed by such wet oxidation method, a gate electrode material is deposited, followed by patterning the gate electrode material by dry etching with a photoresist as a mask. After the photoresist is removed by ashing treatment, the dry etching residue or ashing residue on the surface of the substrate is removed using an etching solution such as hydrofluoric acid.
By the above-described wet etching, the gate oxide film in a region other than that below the gate electrode is etched and,at the same time, the gate oxide film at the side wall end portions of the gate electrode are etched isotropically and undercuts are formed, which brings about inconveniences such as lowering in pressure resistance of the gate electrode. With a view to improving the profile having undercuts formed at the side-wall end portions of the gate electrode, the substrate is subjected to light oxidation treatment, that is, thermal oxidation again to form an oxide film on its surface.
The above-described refractory metals,such as W and Mo are considerably prone to oxidation under a high-temperature oxygen atmosphere so that the application of the above light oxidation treatment to a gate electrode of a poly-metal structure oxidizes the refractory metal film, thereby increasing its resistance or partially peeling off the film from the substrate. In the gate processing by using a poly-metal, it is therefore necessary to take countermeasures against the oxidation of a refractory metal film upon light oxidation treatment.
The above-described Kobayashi discloses a technique of selectively oxidizing Si without oxidizing a W (Mo) film by forming over a Si (silicon) substrate a gate electrode of a W- or Mo-film-containing poly-metal structure and then carrying out light oxidation in a mixed atmosphere of water vapor and hydrogen. This technique makes use of the difference between W (or Mo) and Si in a water vapor/hydrogen partial pressure ratio which brings about equilibrium in redox reaction. In this technique, selective oxidation of Si is realized by setting the partial pressure ratio within a range that W (or Mo) once oxidized by water vapor is reduced rapidly by coexisting hydrogen but Si remains oxidized. The mixed atmosphere of water vapor and hydrogen is generated by the bubbling system which feeds a hydrogen gas in pure water filled in a container and the water vapor/hydrogen partial pressure ratio is controlled by changing the temperature of the pure water.
The above-described Katada and Muraoka disclose a technique of forming a gate electrode of a poly-metal structure, more specifically, forming a metal nitride film such as TiN and a metal film such as W on a Si substrate through a gate oxide film, and then carrying out light oxidation in an atmosphere obtained by diluting a reducing gas (hydrogen) and an oxidizing gas (water vapor) with nitrogen. According to them, only Si can be oxidized selectively without oxidation of the metal film. At the same time, oxidation of the metal nitride film can be prevented, because denitrifying reaction from the nitride metal film can be prevented by diluting the water vapor/hydrogen mixed gas with nitrogen.
As described above, in the step for forming a gate electrode of a poly-metal structure, it is effective for an improvement in the pressure resistance of a gate oxide film and oxidation prevention of a metal film, to carry out light oxidation in a water vapor/hydrogen mixed gas having a predetermined partial pressure.
The conventional bubbling method which has been proposed as a method for forming the water vapor/hydrogen mixed gas is accompanied with the drawback that since the water vapor/hydrogen mixed gas is generated by supplying a hydrogen gas into pure water charged in advance in a container, foreign matters mixed in pure water are fed to an oxidizing furnace together with the water vapor/hydrogen mixed gas and presumably contaminates the semiconductor wafer.
The bubbling method is also accompanied with other drawbacks that since the water vapor/hydrogen partial pressure is controlled by changing the temperature of pure water, (1) the partial pressure ratio varies easily and the optimum pressure ratio cannot be attained with high precision, (2) the water vapor concentration on the order of ppm cannot be attained owing to the controllable range of the water vapor concentration as narrow as several to ten-odd %.
As will be described later, in the redox reaction of Si or a metal by using a water vapor/hydrogen mixed gas, oxidation tends to proceed faster when the water vapor concentration is higher. When Si is oxidized at a comparatively high water vapor concentration as the water vapor/hydrogen mixed gas formed by the bubbling system, the oxide film grows in a markedly short time owing to a high oxidation rate. A minute MOSFET having a gate length not greater than 0.25 μm is required to have a gate oxide film as thin as 5 nm or less in order to maintain the electrical properties of the device. Accordingly, it is difficult to form such a ultra-thin gate oxide film uniformly and with good controllability if the water vapor/hydrogen mixed gas generated by the bubbling method is used. When oxidation is carried out at a low temperature (for example, 80° C. or lower) to reduce the growth rate of the oxide film, a gate oxide film having good quality cannot be obtained.